OBJECTIVE The prevalence of multimorbidity is increasing in many countries due to population aging and other factors. Most studies are cross-sectional, so they do not capture generational differences in multimorbidity. We used longitudinal data over more than 20 years to examine patterns of multimorbidity among cohorts of women of different ages in Australia, and the impact of multimorbidity on their health-related quality of life and use of health services. METHODS We used data from more than 40, 000 participants in the Australian Longitudinal Study on Women's Health, a national cohort study with repeated survey data linked to administrative records of health service use. The women were born in 1989-95, 1973-78 and 1946-51 and followed from 2013 to 2019, 1996 to 2021 and 1996 to 2022, respectively. Main outcome measures Multimorbidity was defined as the cumulative number of chronic conditions common among women aged 18 to 76 years. General practice and pharmaceutical data were obtained from the universal health insurance scheme. Physical and mental health summary scores were calculated from participants' survey responses. RESULTS The conditions that were most prevalent differed between cohorts. Multimorbidity was higher among women born in 1989-95 than among those born in 1973-78 when they were the same age. Health service use and health scores generally increased more with the number of chronic conditions than with age. CONCLUSIONS The composition of multimorbidity changes across the life-course. Generational factors other than age contribute to increases in multimorbidity. Projections of future health service needs must consider these complexities.